Duffle bags are fine. If they are not oversized, you can take them as a carryon.
If you go to the trial/travel size aisle in Target (usually near the pharmacy) you will find travel size shampoo,deoderants, etc.
They also sell a travel pack that has the empty bottles, pump spray bottle and the quart size zipper bag for around $8.99 so you can fill each bottle with your favorite shampoo,etc.
Kids under 12 and adults before 1937 do not have to remove their shoes.
Make it really easy. Go to Costco and buy each child a rollerboard (suitcase that can be used as a carry on that has wheels) $99 GREAT quality

(Kirkland brand)
You will easily be able to fit a pair of shoes, clothes for 5-6 days,etc.
For the second carry on item, give them each a back pack -this can hold the Kindle, a snack, sweatshirt, and anything else they may need.
Then you won't have to worry about checking luggage!